Jinja template syntax

This page is about the Jinja template engine. While the most detailed description of the template language can be found at Jinja documentation, some basic concepts are given in this article.

Code Generation Principle

The code generation is driven by a small script which iterates over the domain model and writes files using the Python Jinja template language.

Given that generator script has read and parsed the IDL file into a domain model, this latter one is then used as the root object for the code generation inside the template language. Below is an example of the code traversing the domain model:

{% for module in system.modules %}
    {%- for interface in module.interfaces -%}
    SERVICE, {{module}}.{{interface}}
    {% endfor -%}
    {%- for struct in module.structs -%}
    STRUCT , {{module}}.{{struct}}
    {% endfor -%}
    {%- for enum in module.enums -%}
    ENUM   , {{module}}.{{enum}}
    {% endfor -%}
{% endfor %}

The template iterates over the domain objects and generates text which is written into a file.

Laguage constructs

Synopsis

A template contains variables and/or expressions, which get replaced with values when a template is rendered; and tags, which control the logic of the template.

There are a few kinds of delimiters. The default Jinja delimiters are configured as follows:

  • {% ... %} for Statements
  • {{ ... }} for Expressions to print to the template output
  • {# ... #} for Comments not included in the template output
  • # ... ## for Line Statements

Control structures

A control structure refers to all those things that control the flow of a program - conditionals (i.e. if/elif/else), for-loops, as well as things like macros and blocks. With the default syntax, control structures appear inside {% ... %} blocks.

For

Loop over each item in a sequence. For example, to display a list of users provided in a variable called users:

<h1>Members</h1>
<ul>
{% for user in users %}
    <li>{{ user.username|e }}</li>
{% endfor %}
</ul>

As variables in templates retain their object properties, it is possible to iterate over containers like dict:

<dl>
{% for key, value in my_dict.iteritems() %}
    <dt>{{ key|e }}</dt>
    <dd>{{ value|e }}</dd>
{% endfor %}
</dl>

Inside of a for-loop block some special variables are available:

VariableDescription
loop.indexThe current iteration of the loop. (starting with 1)
loop.index0The current iteration of the loop. (starting with 0)
loop.revindexThe number of iterations from the end of the loop (starting with 1)
loop.revindex0The number of iterations from the end of the loop (starting with 0)
loop.firstTrue if first iteration.
loop.lastTrue if last iteration.
loop.lengthThe number of items in the sequence.

See for more Jinja documentation

Unlike in Python, it’s not possible to break or continue in a loop. One can, however, filter the sequence during iteration, which allows one to skip items. The following example skips all the users which are hidden:

{% for user in users if not user.hidden %}
    <li>{{ user.username|e }}</li>
{% endfor %}

The advantage is that the special loop variable will count correctly; thus not counting the users not iterated over.

If no iteration took place because the sequence was empty or the filtering removed all the items from the sequence, one can render a default block by using else:

<ul>
{% for user in users %}
    <li>{{ user.username|e }}</li>
{% else %}
    <li><em>no users found</em></li>
{% endfor %}
</ul>

In Python, else blocks are executed whenever the corresponding loop did not break. Since Jinja loops cannot break anyway, a slightly different behavior of the else keyword was chosen.

It is also possible to use loops recursively. This is useful when dealing with recursive data such as sitemaps or RDFa. To use loops recursively, one basically has to add the recursive modifier to the loop definition and call the loop variable with the new iterable where recursion is needed.

The following example implements a sitemap with recursive loops:

<ul class="sitemap">
{%- for item in sitemap recursive %}
    <li><a href="{{ item.href|e }}">{{ item.title }}</a>
    {%- if item.children -%}
        <ul class="submenu">{{ loop(item.children) }}</ul>
    {%- endif %}</li>
{%- endfor %}
</ul>

The loop variable always refers to the closest (innermost) loop. If we there is more than one level of loops, we can rebind the variable loop by writing {% set outer_loop = loop %} after the loop that we want to use recursively. Then, we can call it using {{ outer_loop(...) }}

Please note that assignments in loops will be cleared at the end of the iteration and cannot outlive the loop scope. Older versions of Jinja2 had a bug where in some circumstances it appeared that assignments would work. This is not supported.

If

The if statement in Jinja is comparable with the Python if statement. In the simplest form, one can use it to test if a variable is defined, not empty and not false:

{% if users %}
<ul>
{% for user in users %}
    <li>{{ user.username|e }}</li>
{% endfor %}
</ul>
{% endif %}

For multiple branches, elif and else can be used like in Python. One can use more complex Expressions there, too:

{% if kenny.sick %}
    Kenny is sick.
{% elif kenny.dead %}
    You killed Kenny!  You bastard!!!
{% else %}
    Kenny looks okay --- so far
{% endif %}

Tests

Beside filters, there are also so-called “tests” available. Tests can be used to test a variable against a common expression. To test a variable or expression, its name is used followed by the name of the test. For example, to find out if a variable is defined, one can try name is defined, which will then return true or false depending on whether name is defined in the current template context.

Tests can accept arguments, too. If the test only takes one argument, one can leave out the parentheses. For example, the following two expressions do the same thing:

{% if loop.index is divisibleby 3 %}
{% if loop.index is divisibleby(3) %}

The List of builtin tests can be found on the Jinja documentation page.

Filters

Variables can be modified by functions called filters. Filters are separated from the variable by a pipe symbol (|) and may have optional arguments in parentheses. Multiple filters can be chained. The output of one filter is applied to the next.

For example, {{ name|striptags|title }} will remove all HTML Tags from variable name and title-case the output (title(striptags(name))).

Filters that accept arguments have parentheses around the arguments, just like a function call. For example:

{{ listx|join(', ') }}

will join a list with commas (equivalent to

str.join(', ', listx)

). Nevertheless, the variable filter is applying to is always passed as a first argument to the filter function.

One can define custom filters by registering a Python function as a new filter with the Environment object:

def lower_first_filter(s):
    s = str(s)
    return s[0].lower() + s[1:]


env = Environment(
              loader=FileSystemLoader(search_path),
              trim_blocks=True,
              lstrip_blocks=True
          )
env.filters['lowerfirst'] = lower_first_filter

After that filter called lower first will be available from the template:

{{ var | lowerfirst }}

A list of all supported filters can be found in the Filter Reference.

Variables

Template variables are defined by the context dictionary passed to the template. Variables can be complex object having their own attributes.

One can use a dot (.) to access attributes of a variable in addition to the standard Python __getitem__ “subscript” syntax ([]).

The following lines are equivalent:

{{ foo.bar }}
{{ foo['bar'] }}

If a variable or attribute does not exist, its value will result to undefined value. The interpretation of that kind of value depends on the application configuration: the default behavior is to evaluate to an empty string if printed or iterated over, and to fail for every other operation.

Comments

To comment-out part of a line in a template, use the comment syntax which is by default set to {# ... #}. This is useful to comment out parts of the template for debugging or to add information for other template designers or yourself:

{# note: commented-out template because we no longer use this
    {% for user in users %}
        ...
    {% endfor %}
#}

Line Statements

If line statements are enabled by the application, it’s possible to mark a line as a statement. For example, if the line statement prefix is configured to #, the following two examples are equivalent:

<ul>
# for item in seq
    <li>{{ item }}</li>
# endfor
</ul>
<ul>
{% for item in seq %}
    <li>{{ item }}</li>
{% endfor %}
</ul>

The line statement prefix can appear anywhere on the line as long as no text precedes it. For better readability, statements that start a block (such as for, if, elif etc.) may end with a colon:

# for item in seq:
    ...
# endfor

Line statements can span multiple lines if there are open parentheses, braces or brackets:

<ul>
# for href, caption in [('index.html', 'Index'),
                        ('about.html', 'About')]:
    <li><a href="{{ href }}">{{ caption }}</a></li>
# endfor
</ul>

Since Jinja 2.2, line-based comments are available as well. For example, if the line-comment prefix is configured to be ##, everything from ## to the end of the line is ignored (excluding the newline sign):

# for item in seq:
    <li>{{ item }}</li>     ## this comment is ignored
# endfor

Assignments

Inside code blocks, you can also assign values to variables. Assignments at top level (outside of blocks, macros or loops) are exported from the template like top level macros and can be imported by other templates.

Assignments use the set tag and can have multiple targets:

{% set navigation = [('index.html', 'Index'), ('about.html', 'About')] %}
{% set key, value = call_something() %}

It is not possible to set variables inside a block and have them show up outside of it. This also applies to loops. The only exception to that rule are if statements which do not introduce a scope.

Whitespace Control

In the default configuration:

  • a single trailing newline is stripped if present
  • other whitespace (spaces, tabs, newlines etc.) is returned unchanged

If an application configures Jinja to trim_blocks, the first newline after a template tag is removed automatically (like in PHP). The lstrip_blocks option can also be set to strip tabs and spaces from the beginning of a line to the start of a block. (Nothing will be stripped if there are other characters before the start of the block)

With both trim_blocks and lstrip_blocks enabled, you can put block tags on their own lines, and the entire block line will be removed when rendered, preserving the whitespace of the contents. For example, without the trim_blocks and lstrip_blocks options, this template:

<div>
    {% if True %}
        yay
    {% endif %}
</div>

gets rendered with blank lines inside the div:

<div>

      yay

</div>

But with both trim_blocks and lstrip_blocks enabled, the template block lines are removed and other whitespace is preserved:

<div>
        yay
</div>

One can manually disable the lstrip_blocks behavior by putting a plus sign (+) at the start of a block:

<div>
        {%+ if something %}yay{% endif %}
</div>

It's also possible to strip whitespace in templates by hand. With a minus sign (-) at the start or end of a block (e.g. a For tag), a comment, or a variable expression, the whitespaces before or after that block will be removed:

{% for item in seq -%}
    {{ item }}
{%- endfor %}

This will yield all elements without whitespace between them. If seq was a list of numbers from 1 to 9, the output would be 123456789.

If Line Statements are enabled, they strip leading whitespace automatically up to the beginning of the line.

By default, Jinja2 also removes trailing newlines. To keep single trailing newlines, configure Jinja to keep_trailing_newline.

One must not add whitespace between the tag and the minus sign.

valid:

{%- if foo -%}...{% endif %}

invalid:

{% - if foo - %}...{% endif %}

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